Fractional turn helical antenna

ABSTRACT

A modified helical antenna comprising an array of uniformly spaced fractional turns of a helix mounted on a ground plane bent in the form of a gable such that the fractional turns are positioned outside the ridge of the gable about a common axis parallel to the ridge. The ends of the fractional turns are in electrical contact with the ground plane except for one end of the first fractional turn which passes through the ground plane into a connector. A reflecting plate is positioned perpendicularly to the axis at the end of the antenna opposite the connector to provide a backwardly directed wave which enhances the directivity pattern.

United States Patent Monser Feb. 8, 1972 [54] FRACTIONAL TURN HELICAL ANTENNA [72] lnventorz George J. Mouser, Santa Barbara, Calif.

[73] Assignee: Raytheon Company, Lexington, Mass. 22 Filed: Dec.22, 1969 [21] Appl.No.: 887,024

[s21 u.s.c1. .1; ..f. ..'..343/s9s,343/70s 51 1m. 01.. 9 3;; 58] Field ofSearch ..343/846, 848, 895, 90s, 70s, I I 343/708 [56] References emu UNITED STATES PATENTS 3,066,295 11/1962 Krause et al .1 ..34a/s95 3,503,075

3/1970 Gerst ..343/895 3,249,946 5/1966 Flanagan ..343/792.5 3,5S0,l36 12/1970 Walter ,et al. ..343/895 Primary Examiner-Eli Lieberman Att0mey--Philip J. McFarland and Joseph D. Pannone ABSTRACT A modified helical antenna comprising an array of uniformly t spaced fractional turns of a helix mounted on a ground plane bent in the form of a gable such that the fractional turns are positioned outside the ridge of the gable about a common axisparallel 'to the ridge. The ends of the fractional turns are in electrical contact with the ground plane except for one end of the first fractional turn which passes through the ground plane into a connector. A reflecting plate is positioned perpendicularly to the axis at the end of the antenna opposite the connector to provide a backwardly directed wave which enhances the directivity pattern.

4 Claims, 2 Drawing Figures mimmrm'awn 3.641580 INVENTOR GEORGE J. MONSER FRACTIONAL TURN HELICALANTENNA BACKGROUND OF THE INVENTION craft. First, the presence of the metallic reflecting surface of I the aircraft may distort the desired directivity pattern of the antenna. Secondly, there is frequently a lack of space for the electronic equipment such as transmitters and receivers which connect with the antenna. r

Accordingly it is an object of the invention to provide an antenna having electrical characteristics approximating those of the helical antenna and adapted to be'mounted on the metallic surface of avehicle such as an aircraft.

Another object of the invention is to provide an antenna having electrical characteristics approximating those of the helical antenna and having sufficient rigidity to be operated on board an aircraft. v

A further object of the invention is to provide an antenna having electrical characteristics approximating those of the helical antenna an adaptedto minimize the space required for storage of ancillary electronics equipment within a vehicle carrying the antenna.

SUMMARY OF THEINVENTION BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and other features of the invention are explained in the following description taken in connection with the accompanying drawing wherein:

FIG; 1 is an enlarged pictorial representation of the invention shown mounted on the underside of an aircraft; and

FIG. 2 is an isometric view, partially in block diagram'form, of the antenna in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I there is shown an antenna 20 of the invention mounted on the underside of aircraft 22 and positioned to generate a beam 24 of radiation in the forward direction. The antenna half power (3 db.) beamwidth varies over the range of 70-95 as the frequency is varied over a band of approximately 35 percent of the center frequency of the radiation. The antenna gain is 4-5 db. relative to an isotropic radiator. The polarization of the electric field is vertical when the antenna 20 is mounted as shown in FIG. 1. (This polarization differs from that of the usual helical antenna of the prior art wherein the polarization is circular about the helix axis.) It is understood that the antenna 20 may also be mounted in other positions relative to the aircraft thus altering the direction of the beam 24 and the polarization. The antenna 20 connects with electronic equipment 26 which is positioned on the surface of the aircraft 22 and enclosed by the antenna in a manner to be described. In a typical application the electronic equipment 26 may include receiver and transmitter equipments which are coupled to the antenna 20.

As shown in FIG. 1 and in greater detail in FlG..2, the antenna 20 comprises. a plate bent in the form of a gable 28. a disk 30 mounted at an end of the gable opposite the direction of the beam 24, and a plurality of elements 32 external to the comer of the gable 28. Each of the elements 32 has the form of a fractional turn of a helix and supports a traveling wave along the ridge of gable 28. One of the elements designated 32A connects with electronic equipment 26 in a manner to be described while the other elements 32 are parasitically excited. The elements 32 are uniformly spaced, preferably one-fourth wavelength apart at the midband frequency along an axis parallel to and preferably coinciding with theridge of the gable 28. The spacing of the disk 30 from the nearest of the elements 32 is not critical but is preferably one-eighth wavelength. The gable 28, disk 30 and elements 32 are each constructed from an electrically conductive material such as copper which provides both the desired electrical characteristics, as well as sufficient rigidity to be utilized on an aircraft.

The gable 28 provides in cooperation with the metallic surface of the aircraft 22, a ground plane for the antenna 20. Each element 32 is rigidly connected at both its ends, as by soldering, to the gable 28 except-for one element 32A which has only one of its ends connected to gable 28 while its other end is insulated from the gable. 28 by connector 34. Connector 34, which is preferably a coaxial connector, provides access for electrical signals between element 32A located outside the gable 28 and the electronic equipment 26 located within the gable 28.

' It is noted that the form of the antenna 20 is particularly well adapted for aircraft use in that it provides space for the electronic equipment 26 within the gable 28 so that there are reduced requirements for both valuable space within the aircraft 22 as well as means for cooling the electronic equipment 26. It is also noted that many antennas of the prior art do not operate effectively in the vicinity of a large ground plane such as the metallic surface of an aircraft since this ground plane distorts the directivity patterns of these antennas. In contradistinction, the antenna 20 is specifically designed about a ground plane, the gable 28, which cooperates with the metallic surface of the aircraft 22 to form the beam pattern. This antenna design provides the well-known characteristics of broad bandwidth and beamwidth of 'a helical antenna with a structure that is more rigidly grounded than a helical antenna. In addition the disk 30 serves as a reflecting plate whereby energy traveling along the antenna structure away'from the element 32A is reflected back to increase the intensity of the beam 24. In the absence of the disk 30, the antenna 20 radiates both forwards and aft along the antenna axis with the major lobe of the directivity pattern being at the end nearest the element 32A. (Note that with the usual helical antenna of the prior art, the major lobe would be in the opposite direction.)

A model of the invention built to operate in the frequency range of 2.5 to 3.5 gI-lz. has the following dimensions:

Length of gable 28 6 inches Angle at corner (or ridge) of gable 28 90 Slant height of gable 28 2.1 inches Number of elements 32 6 Spacing of elements 32 it inch Spacing of disk 30 from nearest element 32 56 inch Element 32 wire gauge (Diameter) lit inch Element 32 turn diameter llfit inch Disk 30 diameter 2 inches Connector 34 Type N" female It is understood that the above described embodiment of the invention is illustrative only and that modifications thereof will occur to those skilled in the art. Accordingly, it 's desired that this invention is not to be limited to the embodiipent disclosed herein but is to be limited only as defined by the appended claims.

Whatisclaimedis:

ground plate on opposite sides of the ridge of the gable; which is parallel to the ridge of the gable; and

and connector means coupling with an element whereby energy connector means coupling with an element whereby energy is communicated between the antenna and a source of is communicated between the antenna and a source of energy.

energy. 5 3. The antenna of claim 2 wherein only the element at one 2. An antenna comprising: end of'the row of elements is coupled to the connector means. a ound l te b t i he f ()fa bl 4. The antenna of claim 3 wherein a reflector is positioned a plurality of elements spaced apart from each other, each beyond the end of the row of elements pp the radiator element having the form of a fractional turn of a helix, which is coupled to the Connecting means the reflector lO necting with the ground plate.

II i '8 II I each element being rigidly affixed to the ground plate, the elements being arranged in a row about a common axis 

1. An antenna comprising: a ground plate bent in the form of a gable; a plurality of elements spaced apart from each other, each element having the form of a fractional turn of a helix, the two ends of each element being rigidly affixed to the ground plate on opposite sides of the ridge of the gable; and connector means coupling with an element whereby energy is communicated between the antenna and a source of energy.
 2. An antenna comprising: a ground plate bent in the form of a gable; a plurality of elements spaced apart from each other, each element having the form of a fractional turn of a helix, each element being rigidly affixed to the ground plate, the elements being arranged in a row about a common axis which is parallel to the ridge of the gable; and connector means coupling with an element whereby energy is communicated between the antenna and a source of energy.
 3. The antenna of claim 2 wherein only the element at one end of the row of elements is coupled to the connector means.
 4. The antenna of claim 3 wherein a reflector is positioned beyond the end of the row of elements opposite the radiator which is coupled to the connecting means, the reflector connecting with the ground plate. 